Can you sell me a complete kit? An assembled unit?

Nope. You gotta collect the parts and build it for yourself. Relax–it’s not hard, and you might learn something. If you get stuck, leave a comment below and I’ll do my best to help.

Where do I get the parts?

FAR Circuits sells a printed circuit board and a programmed PIC chip for this project. Go here for ordering details. The rest of the parts can be purchased from nearly any electronics supply house. I use Jameco and Mouser. A parts list with part numbers for Jameco and Mouser is on my how-to-build page.

What encoders should I use?

I recommend the S5S or S6S encoders from U.S. Digital. You don’t need any of the options like index, ball bearings, etc. Encoders are rated in counts per revolution (CPR), but because we use both channels in quadrature mode, the actual resolution (what I call tics/rev) is four times the CPR. So, encoders rated at 1024 CPR will give us 4096 tics/rev, or about 0.09 degrees per tic (assuming a gear ratio of 1:1 when attached to the telescope). I recommend choosing encoders that give somewhere between 4000 and 10,000 tics per revolution of the telescope on its axis, taking into account any gear ratio due to the way you attached the encoders to your telescope. The maximum allowed tics/rev is 65535, but 4000 to 10,000 tics/rev is good enough.

Can I use encoders from a mouse?

Some people have done that successfully, but I can’t tell you how to do it–you’ll have to figure it out for yourself. The interface is expecting each encoder to have two channels that output 0V and 5V as the encoder is turned. Mouse encoders are fairly low resolution, too, so you’ll need some gearing.

How do I attach the encoders to my telescope?

If you have a dob, I recommend attaching the encoders directly to the two telescope axes. For other types of mounts, you’re on your own. There are too many different types of mounts for me to be able to tell you how to do it. Just remember two things: the encoders must turn whenever the telescope moves, regardless of whether it’s being motor driven or pushed around by hand, and there must not be any encoder slippage or backlash.

How do I figure out my encoder resolution?

Multiply the encoder’s CPR rating by four, and then by the gear ratio (if any) by which it’s attached to the telescope.

What kind of serial cable do I need?

You need a straight-through cable, not a null-modem cable (unless you’re using a PDA instead of a PC–see below for details).

My computer doesn’t have a serial port. How can I connect the interface?

You’ll need a USB-to-serial converter. I’ve been told that not all of them work very well, so you might want to do a little research. If you find one that works, leave a comment below and I’ll add it to the FAQ.

What software works with your interface?

Does your interface emulate other types of interfaces?

Yes, it emulates the various Tangent Instruments interfaces (BBox, NGC-MAX, etc.). It also emulates David Lane’s Microguider III.

Can I use your interface with a PDA instead of a PC?

Yes, if you have a serial cable for your PDA. A USB cable will not work, even with a USB-to-serial adapter. You’ll also need a null-modem adapter in order to be able to plug in the PDA’s serial cable. See my project description page for details and a special note about Handspring Visors.

How can I bench-test the interface? Must I have the encoders connected?

You can bench-test the interface without the encoders connected to verify that the communication with the PC is working. See the Testing section of my how-to-build page for details.

Why do both the RA and declination change in my software even if I only turn one encoder?

This happens because your telescope is not perfectly polar-aligned. Don’t worry about it. As long as you’re finding objects, everything’s working fine. This often throws people when they are bench testing using my Windows software. It’s natural to assume that only one number should change when only one encoder is turned, but there’s a lot of math going on to convert the encoder counts to celestial coordinates.

I’ve built the interface but it doesn’t work. What do I do now?

Follow the procedures listed in the Testing section of my how-to-build page.

Can you add an LCD display to your interface and eliminate the need for a PC?

No. I get asked this a lot. The PIC chip isn’t powerful enough to handle the math needed for something like this. I’d need to replace it with a full-fledged microprocessor, and I don’t know how to do that. Sorry.

21 thoughts on “Frequently-Asked Questions”

Hi Dave,
I have a friend who has made a 12″ f/5 Dob and would like to know if your unit will work if his scope is mounted on an equatorial platform?

BTW my Ek box is now 5 years old and gets used every clear night on both by 12: f/6 and 8″ f/6 Dobs. Everyone who sees it wants one!
I use a Keyspan serial to USB adapter available from Apple stores here in Australia. You could probably obtain one from Amazon I guess as well. They’re not cheap but they work a treat.

hello world..
a question , Ilike to build this system on my EQ6. Arvis showed me how,m but the DEC gearing wheel is in size smaller, innerside 73 mm instead 86,5 mm for the RA gear wheel, is this to adjust in the software? does anyone know where I could order these gear wheels, not the whole set, I allready have the encoders.
Greetings from Zwolle Netherlands.

I’d like to build this setup for my Celestron CG-4 mount. I’ve studied your site and feel I can handle all of it, save for one point that I’m not experienced with… gear ratios. I saw photos from http://www.wildcard-innovations.com.au/images/enc_cg4.gif that show how to mount for the declination encoder and assume that the roller size in the photo has a lot to do with that – Any advice for how to compute the resolution for such a setup? Also, do you feel it would be possible to mate your design to the CG-4? Thanks in advance and I’m stoked to find such a cool home-brew setup like this – looks like a fun project.

Jon, in a case like that I’d be inclined to measure the resolution instead of calculate it. I’d do that by using something like Hyperterminal to manually monitor encoder readings (as described in the Testing section of my Building the Circuit page).

Configure Hyperterminal for 9600 baud, 8 bits, 1 stop bit, no parity, no flow control. Note that the DSC interface doesn’t echo typed characters back to Hyperterminal, so you’ll be typing blind. Also, it never sends line feeds, so when you query it for encoder positions it’ll overwrite whatever’s currently displayed.

The commands you’ll need to type into Hyperterminal are (type what’s inside the quotes, but don’t type the quotes themselves):

To set the encoder resolution: “Z +65535 +65535” ( means hit the Enter key). The interface should respond with an asterisk
To check the encoder resolution: “H” The interface should respond with “+65535 +65535”.
To check the encoder positions: “Q” The interface should respond with “+XXXXX +XXXXX” where XXXXX are the positions.

NOTE: the H and Q commands will always respond with a sign (plus or minus) and five digits. If it appears that there is a sixth digit, ignore it–it’s because the output from a previous command didn’t get entirely overwritten.

Okay, here’s theh process for measuring encoder resolutions:

1) Set the encoder resolutions to their highest value (65535).
2) with a high-power eyepiece, point the telescope at a distant object, centering it in the view. Record the encoder readings for this position.
3) Carefully rotate the telescope on one of its axes through one complete revolution until the distant object is once again centered in the view. Record the encoder readings again.
4) Repeat the process for the other telescope axis.

For each axis, only one encoder reading should change as the telescope is rotated on that axis. You’ll want to stop periodically during the process of rotating so that you sense whether the encoder readings get larger or smaller. The readings will be in the range from -32767 to +32767, so you’ll need to know whether the way you rotate your telescope on its axis makes the encoder readings increase or decrease. The resolution will be the change in encoder readings over one complete revolution.

As far as the CG-4 mount goes, I can’t really make a recommendation on how to mount the encoders–I’m not familiar with the mount–but it should be possible. You’ll probably have to drill some mounting holes in your mount and fashion some brackets to hold the encoders. You’ll also need to find a wheel or something that can turn the encoder shaft when the telescope is rotated on its axes. The important thing to know is that *any* movement of the telescope on an axis needs to result in that axis’s encoder turning.

Got you on that Dave – thanks so much for this procedure and the advice. I think I can make it work and will definitely take photos of my progress. Am in the process of building my observatory right now – digging the pier and getting the building done in the next two months and am researching my pier and mount options in my spare time to be sure I know where I’m going with this. Appreciate your help immensely. — Jon

Dave I know this is not exactly related to your DSC box but perhaps someone has presented this situation to you before. I have my encoders mounted on a Meade SCT model LX3/2120. The RA encoder is mounted in the base of the fork and coupled directly to the center shaft. Consequently when the fork is rotated manually the encoder rotates and moves the RA reading as it should. The problem is that when the RA motor is running and turning the fork the encoder still rotates and moves the RA reading. When the scope is pointed at a star the star’s RA does not change so when the scope is tracking it across the sky the RA encoder should not rotate or it will change the indicated RA in the DSC readout. I hope I have explained this adequately and you know how others have worked this out. Thanks

Pat, your telescope is set up correctly. The software that monitors the positions of the encoders is aware that time is passing and takes that into account when using the encoder positions to calculate the coordinates at which the telescope is aimed. Think of the inverse case: if the RA encoder wasn’t turning as the telescope is tracking, the software would have to be told that the telescope was tracking in order to know that the RA isn’t changing. If you turned the tracking off, the telescope’s RA would be changing with time as the sky rotated overhead but the encoder still wouldn’t be turning, and the software monitoring the encoder wouldn’t see any change.

Another disadvantage to having the RA encoder not turn when the telescope is tracking is that you then have to assume that the RA does not change as the telescope tracks. Theoretically, the RA would stay the same only if your telescope mount had no imperfections or inaccuracies *and* if it was aligned perfectly to the pole. Realistically, this would never be the case, so it’s better if the RA encoder always moves when the telescope moves (whether driven or not). That way, no such assumptions need to be made.

It’s worth noting that the software packages that monitor the encoder positions and calculate where the telescope is pointing (like my ASCOM driver, for example) do not make any assumptions about whether the mount is accurately polar-aligned. The algorithms I use don’t even care whether the mount is an alt-az or an equatorial, or whether it’s being driven or not, as long as the encoders turn whenever the telescope turns. The initial alignment process establishes the relationship between the telescope’s coordinate system and the celestial coordinates for any arbitrary orientation of the telescope mount. I could plop my equatorial mount down in the back yard without even trying to polar align, and as long as I go through the alignment process, the software can tell me where I’m pointing. (My telescope wouldn’t be very convenient to use if it wasn’t polar aligned, but the encoder system would still work.)

Dave, Will a pair of Nidec Nemicon magnetic rotary encoders (NOC-S1000-2MD) with a resolution of 1000 p/r work the decoder project? I know you recommend the S5S or S6S encoders from U.S. Digital which are both optical encoders and around $85 – $100 each. Even though the magnetic encoders are a bit bigger (2″ dia x 1.3″ deep), they are less than 1/2 the price.

Rick, they might possibly work. The DSC board runs at 5 V and the minimum supply voltage for these encoders is 4.5 V. It’s not clear what the high-state output voltage of these encoders is, but it needs to be probably 3.5V or greater. I couldn’t say for sure whether these will work without trying one. If you decide to go that route, let me know how they work out.

Encoder errors are when the circuit misses one or more counts from the encoder itself. The microcontroller periodically checks the values of the A and B channels coming from the encoder and expects them to change in a certain sequence, indicating which way the encoder is turning. Only one of the two channels (if any) should have changed value between each check. The checks occur a few thousand times per second, frequently enough to catch each change as long as the encoder isn’t spinning really fast (unlikely when connected to a shaft of the telescope mount). But if, say, both the A and B channels change values between checks by the microcontroller, that would result in an encoder error because that means the microcontroller missed one of the changes and now cannot tell which way the encoder turned. Thankfully, this pretty-much never happens, so don’t worry about it.